In a spark ignition internal combustion engine, ignition discharge is effected at a spark plug by an ignition device including an ignition coil and the like, and fuel, guided into a combustion chamber, is burned by this ignition discharge. To make the combustion condition more desirable, multiple discharge is proposed to effect ignition discharges at a spark plug more than once in one combustion stroke of each cylinder. Thus, ignition discharge is repeatedly effected at a spark plug during a predetermined multiple discharge period.
For example, U.S. Pat. No. 5,056,496 (JP 2811781) discloses an ignition system, which is a combination of a capacitive discharge ignition device and a multiple discharge ignition device. In this system, a battery, an energy storage coil, and a first switch are connected in series, and the energy storage coil, a reverse-flow prevention device, a primary coil of an ignition coil, and a second switch are connected in series. The energy storage coil is connected with a capacitor through the reverse-flow prevention device, and the secondary coil of the ignition coil is connected with a spark plug.
With this construction, after the energy storage coil and the capacitor are charged, the energy storage coil and the capacitor are discharged to charge the ignition coil. At the same time, initial ignition discharge is effected at the spark plug. Thereafter, the first and second switches are periodically and alternately turned on and off. Thus, the energy storage coil is charged and the ignition coil is discharged. Meanwhile, the ignition coil is charged and the energy storage coil is discharged. Thus, a current is supplied through the secondary side of the ignition coil both in the forward direction and in the reverse direction to effect ignition discharge repeatedly at the spark plug. Multiple discharge is thereby carried out.
Some of recent engines have been improved to increase in-cylinder flow velocity of air-fuel mixture gas for the improvement of combustion condition. In such a case, the flow velocity of gas around the spark plug is increased, and this increases a voltage required to hold ignition discharge. In an ultra-lean burn engine or the like with its air-fuel ratio significantly made lean, the flow velocity of gas around a spark plug is further increased. It is required to supply a large current through the spark plug. In such an environment, a required secondary current cannot be ensured.
In addition, since the flow velocity of gas in proximity to a spark plug is high in such engines, ignition discharge can vanish during multiple discharge. To cope with this, a power supply system connected to the primary coil is provided with a booster circuit such as a DC/DC converter. However, in the booster circuit, variation is produced in coil inductance or the like due to an individual difference, change over time, or the like, which can lead to excess or deficiency in energy supplied to the primary coil of the ignition coil. If there is produced excess or deficiency in energy supplied to the primary coil, that makes spark discharge at a spark plug unstable.